The most vital element on earth is oxygen. Without it, human life simply could not exist. Oxygen is about 21% by volume (about 23% by weight) of our natural atmosphere, regardless of climate, altitude and geography. It is believed that in the past, atmospheric oxygen levels were higher, perhaps significantly so, than today. The decrease in oxygen content of ambient air during the last couple of centuries may be a result of reductions in plant life and/or increases in the use of fossil fuels, among other factors. Oxygen levels may have exceeded 35% in prehistoric times.
There is little doubt that lower oxygen levels impair physical and mental function. In urban areas and enclosed spaces, oxygen levels can be substantially lower than 21%. In high altitudes, relative oxygen concentration is the same but all gases are rarefied, which is equivalent to lower oxygen levels.
The level of carbon dioxide (CO2) in our blood triggers breathing, not the amount of oxygen. Humans are not very good at compensating for low or high oxygen levels by breathing more or less. The benefit of higher oxygen concentration is that getting more oxygen, at least some of the time, is highly desirable. Medical treatment for intensive care and for very sick patients routinely uses high purity oxygen.
Hyper baric oxygen treatment is a very common form of alternative medicine believed by many to have broad health benefits ranging from aging and aches to cancer and infectious diseases. Oxygen bars have cropped up that offer healthy patrons pure oxygen inhalation. There are books and groups espousing the therapeutic value of increased oxygen intake and its critical role in a vast array of common ailments.
Oxygen separation is a well-established technology. Purified oxygen is the third largest bulk chemical market. It is of critical importance in industrial processes, medicine, research and development and aquaculture. In the future, purified oxygen could have dramatic positive impact on energy generation and reduction of air pollution.
Several technologies exist for separating oxygen from air. One technology is Pressure Swing Adsorption (PSA) devices which are used for smaller, on-site applications. This technology was originally developed in the 1950s-1960s. Another technology is cryogenic separation, which is often used, in large industrial facilities.
New oxygen separation technologies are being developed. There are significant ongoing efforts in industry and academic research to find cheaper and more efficient ways to separate pure oxygen. Most of these methods are based on diffusion or filtering through new materials such as ceramics, membranes, etc. All of these efforts target existing industrial and medical applications. A particular important target is making it economically feasible to use enriched oxygen in burning fossil fuels.
Many homes and buildings have central heating, ventilation and air conditioning (HVAC) systems, among whose function is to maintain air quality. HVAC systems typically re-circulate much of the internal air while also replacing some of it with fresh air from the outside. While this helps dilute internal contaminants, it represents an additional thermal load and in some cases may also introduce external pollution. It also may dilute away desirable components of air whose concentration has been elevated, such as humidity or oxygen.
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) has set some fairly detailed guidelines for the rate of replacement of air in confined spaces under various conditions, which can be found in their publications.